Heat butter in a large frying pan. Add onion, garlic and ginger and cook slowly until just beginning to turn golden. Add curry powder and paprika and stir for 2-3 minutes. Add salt, lemon juice and coconut milk and simmer until thick, stirring occasionally. Add prawns. If using raw prawns, simmer until they turn pink, about 5 minutes. Cooked prawns will take only 2-3 minutes to heat through. Sprinkle with toasted coconut. Serve with boiled rice.